Morphologic interaction of bridges during high-energy flood events by the example of the flood of mid-July 2021 in the Ahr Valley, Germany

In mid-July 2021, heavy rainfall led to severe flash floods in the Eifel-mountain region in western Germany. The Ahr River, a tributary to the Rhine River in Rhineland-Palatinate, was most severely affected. Discharges accumulated rapidly in the narrow valley and formed a fast-moving flood wave, leading to record-breaking water levels. High hydraulic forces in combination with driftwood and debris accumulation led to local back-up, and eventually failure of many bridges [1].

The flood is proven to be a high-energy event that led to significant morphologic changes [2]. Besides lateral river course changes, severe local erosion was observed after the flood, and especially documented near bridges [3].

The Ahr Valley is well suited for a case study on the morphologic interaction of bridges with high-energy floods, as around 75% of the 114 bridges were damaged or destroyed in 2021 [1]. In April 2022, bathymetry data was collected at two affected bridges. Further, selected bridges were monitored by drone surveys every six months, allowing us to investigate the morphologic development over the last two years.

Besides bridge overtopping , pier scouring led to structural failure [1, 4, 5]. At a railway bridge between Reimerzhoven and Dernau (N: 50.532213, E: 7.062320) scouring occurred upstream on the pier in the middle of the riverbed, and bank erosion occurred on the right-hand in flow direction. Riverbed narrowing and sediment deposition at the remaining piers occurred on a small scale. Further upstream, at the town of Altenahr, massive debris accumulation, bridge overtopping, and pier scouring downstream of the middle pier led to structural failure. At this location, a railway bridge and a trafficable bridge were located next to each other (N: 50.514921, E 6.985825). The railway bridge experienced severe bank erosion on the left-hand side in flow direction. After removal, shoal formation led  to the cut-off of a stagnant water pool [2]. Processes of pier scouring, bank erosion, and sediment deposition in the following two years differ in both examples. Further investigation and comparison help to gain an understanding of the complex morphologic interaction of bridges in high-energy flood events. Parameters, like the main flow direction and the local flow velocity as well as driftwood accumulation leading to bridge overtopping impact patterns of erosion and deposition. Results can support local water resources management as well as bridge construction authorities.

[1] Burghardt L, Schüttrumpf H, Wolf S et al. (2022) Analyse der Schäden an Brückenbauwerken in Folge des Hochwassers 2021 an der Ahr. Wasser Abfall 24:12–17

[2] Wolf S, Stark N, Holste I et al. (2023) Evaluation of the High-Energy-Flood of mid-July 2021 as a Morphologic Driver in the Ahr Valley [preprint]

[3] Lehmkuhl F, Keßels J, Schulte P et al. (2022) Beispiele für morphodynamische Prozesse und Verlagerungen in Folge des Hochflutereignisses 2021 im Ahrtal. Wasser Abfall 24:40–47. https://doi.org/10.1007/s35152-022-1349-7

[4] Pucci A, Eickmeier D, Sousa HS et al. (2023) Fragility Analysis Based on Damaged Bridges during the 2021 Flood in Germany. Applied Sciences 13:10454. https://doi.org/10.3390/app131810454

[5] Lemnitzer A, Stark N, Gardner M et al. (2022) Geotechnical Reconnaissance of the 2021 Western European Floods. GEER Association (Report - 76)